The present invention is generally related to the fabrication of integrated circuits (ICs) and, more specifically, to the fabrication of a conductive electrode barrier using a composite film of iridium (Ir), tantalum (Ta), and oxygen.
Platinum (Pt) and other noble metals are used in IC ferroelectric capacitors. The use of noble metals is motivated by their inherent chemical resistance. This property is especially desirable under high temperature oxygen annealing conditions, such as those seen in the fabrication of ferroelectric capacitors. In addition, chemical interaction between noble metals and ferroelectric materials such as perovskite metal oxides, is negligible.
Specifically, the above-mentioned noble metals are used as conductive electrode pairs separated by a ferroelectric material. One, or both of the electrodes are often connected to transistor electrodes, or to electrically conductive traces in the IC. As is well known, these ferroelectric devices can be polarized in accordance with the voltage applied to the electrode, with the relationship between charge and voltage expressed in a hysteresis loop. When used in memory devices, the polarized ferroelectric device can be used to represent a "1" or a "0". These memory devices are often called ferro-RAM, or FRAM. Ferroelectric devices are nonvolatile. That is, the device remains polarized even after power is removed from the IC in which the ferroelectric is imbedded.
There are problems in the use of metal, even noble metal electrodes. Pt, perhaps the widely used noble metal, permits the diffusion of oxygen, especially during high temperature annealing processes. The diffusion of oxygen through Pt results in the oxidation of the neighboring barrier and substrate material. Typically, the neighboring substrate material is silicon or silicon dioxide. Oxidation can result in poor adhesion between the Pt and neighboring layer. Oxidation can also interfere with the conductivity between neighboring substrate layers. Silicon substrates are especially susceptible to problems occurring as a result of oxygen diffusion. The end result may be a ferroelectric device with degraded memory properties. Alternately, the temperature of the IC annealing process must be limited to prevent the degradation of the ferroelectric device.
Various strategies have been attempted to improve the interdiffusion, adhesion, and conductivity problems associated with the use of noble metals as a conductive film in IC fabrication. Titanium (Ti), titanium oxide (TiO.sub.2), and titanium nitride (TiN) layers have been interposed between a noble metal and silicon (Si) substrates to suppress the interdiffusion of oxygen. However, Ti layers are generally only effective below annealing temperatures of 600 degrees C. After a 600 degree C. annealing, Pt diffuses through the Ti layer to react with silicon, forming a silicide product. Further, the Pt cannot stop the oxygen diffusion. After a high temperature annealing, a thin layer of silicon oxide may be formed on the silicon surface, which insulates contact between silicon and the electrode.
Other problems associated with the annealing of a Pt metal film are peeling and hillock formation. Both these problems are related to the differences in thermal expansion and stress of Pt with neighboring IC layers during high temperature annealing. A layer of Ti overlying the Pt film is known to reduce stress of the Pt film, suppressing hillock formation.
Ir has also been used in attempts to solve the oxygen interdiffusion problem. Ir is chemically stable, having a high melting temperature. Compared to Pt, Ir is more resistant to oxidation and oxygen diffusion. Further, even when oxidized, iridium oxide remains conductive. When layered next to Ti, the Ir/Ti barrier is very impervious to oxygen interdiffusion. However, Ir can diffuse through Ti. Like Pt, Ir is very reactive with silicon or silicon dioxide. Therefore, a bilayered Ir/Ti or Ir/TiN barrier is not an ideal barrier metal.
Co-pending application Ser. No. 09/263,595, entitled "Iridium Conductive Electrode/Barrier Structure and Method for Same", invented by Zhang et al., and filed on Mar. 5, 1999, discloses a multilayered Ir/Ta film that is resistant to interdiffusion.
It would be advantageous if alternate methods were developed for the use of Ir as a conductor, conductive barrier, or electrode in IC fabrication. It would be advantageous if the Ir could be used without interaction to an underlying Si substrate.
It would be advantageous if an Ir film could be altered to improve interdiffusion properties. Further, it would be advantageous if this improved type of Ir film could be layered with an interposing film to prevent the interaction of Ir with a silicon substrate. It would be advantageous if the multilayered film including a layer of Ir could resist the interdiffusion of oxygen at high annealing temperatures. It would also be advantageous if the multilayered film including Ir was not susceptible to peeling problems and hillock formation.
It would be advantageous if an altered Ir film could be produced which remains electrically conductive after annealing at high temperatures and oxygen ambient conditions.
Accordingly, a highly temperature stable conductive barrier layer for use in an integrated circuit is provided. The barrier comprises an underlying silicon substrate, a first barrier film including tantalum (Ta) overlying the substrate, and an iridium-tantalum-oxygen (Ir--Ta--O) composite film overlying the first barrier film. The Ir--Ta--O composite film remains conductive after high temperature annealing processes in an oxygen environment. Further, the Ir composite film resists hillock formation, and resists peeling.
In some aspects of the invention a second barrier film, including a noble metal, overlies the Ir--Ta--O composite film. The second barrier film improves the Ir--Ta--O interface to subsequently deposited layers, and limits the diffusion of oxygen into the Ir--Ta--O film.
Typically, the first barrier film is selected from the group of materials consisting of Ta, tantalum silicon nitride (TaSiN), and tantalum nitride (TaN). The first barrier layer has a thickness in the range of approximately 10 to 100 nanometers (nm).
In some aspects of the invention, the barrier is used to for an electrode in a ferroelectric device. Then, a ferroelectric film overlies the Ir--Ta--O film. Alternately, the second barrier layer intervenes between the Ir--Ta--O film and the ferroelectric film. A conductive metal film made of a noble metal, the above-mentioned Ir composite film, or other multilayered conductive top electrode overlies the ferroelectric film. The ferroelectric film is capable of storing charges between said top and Ir--Ta--O electrodes.
Specifically, the Ir composite film includes the following materials. Either Ir, Ta, and oxygen, or Ir, Ta, and IrO.sub.2, or Ir, Ta, and Ta.sub.2 O.sub.5, or IrO.sub.2 and Ta.sub.2 O.sub.5, or IrO.sub.2, Ta.sub.2 O.sub.5, Ir, and Ta, or Ir and Ta.sub.2 O.sub.5, or Ta and IrO.sub.2, or IrO.sub.2, Ta.sub.2 O.sub.5, and Ir, or IrO.sub.2, Ta.sub.2 O.sub.5, and Ta. Further, the above-mention Ir composite film groups are intended to include gamma-phase variations of Ta.sub.2 O.sub.5 and (Ta, O). Typically, the Ir--Ta--O composite film has a thickness in the range of approximately 10 to 500 nm.
Also provided is a method for forming a highly temperature stable conductive barrier layer overlying an integrated circuit substrate, the method comprising the steps of:
a) depositing a first barrier layer including tantalum overlying the substrate; and PA1 b) depositing an iridium-tantalum-oxygen composite film overlying the first barrier layer, whereby a multilayer structure is formed that is resistive to interaction with the substrate. PA1 c) depositing a second barrier layer including a noble metal overlying the Ir--Ta--O composite film, whereby the second barrier layer resist the diffusion of oxygen into the Ir--Ta--O film, improving the Ir--Ta--O film interface to subsequently deposited materials. The second barrier film selected from the group consisting of Ir, Ru, IrO.sub.2, platinum (Pt), and RuO.sub.2, and has a thickness in the range of approximately 10 to 200 nm. Step c) includes depositing the second barrier layer through deposition methods selected from the group consisting of physical vapor deposition (PVD), chemical vapor deposition (CVD), and metal organic CVD (MOCVD). PA1 d) depositing a ferroelectric material overlying the Ir--Ta--O composite layer; and PA1 e) depositing a conductive top electrode overlying the ferroelectric material, whereby a ferroelectric capacitor is formed.
The Ir--Ta--O composite film and first barrier layer are deposited through by deposition methods selected from the group consisting of physical vapor deposition (PVD), chemical vapor deposition (CVD), and metal organic CVD (MOCVD). In some aspects of the invention Step b) includes cosputtering both Ir and Ta targets at a power in the range of approximately 200 to 400 watts, with the power on the Ta target greater than, or equal to the power on the Ir target, and in which the Ar to O.sub.2 ratio is approximately 1:X, where X is greater than, or equal to 1.
Some aspects of the invention include a further step, following Step b), of:
When a ferroelectric capacitors is to formed further steps follow Step c), of: